Standard light source



Aug.'4, Z1970 E G. L. FOUGERE 3,

7 STANDARD LIGHT SOURCE Filed Aug. 17, 19.67

VOLTAGE REFERENCE SERVO AMPLIFIER Fig.5

INVENTOR. Guy L. Fougere Attorney United States Patent 3,522,464 STANDARD LIGHT SOURCE Guy L. Fougere, Lincoln, Mass., assignor to Arthur D. Little, Inc., Cambridge, Mass., a corporation of Massachusetts Continuation-impart of application Ser. No. 297,061, July 23, 1963. This application Aug. 17, 1967, Ser. No. 661,337

Int. Cl. H01j 1/52, 5/16; H01k I/26 US. Cl. 313-117 2 Claims ABSTRACT OF THE DISCLOSURE This application is a continuation-in-part of my copending application Ser. No. 297,061, filed July 23, 1963, now abandoned.

In the use of analytical instruments, e.g. spectrometers, spectrophotometers, flame photometers, and other spectroscopic devices, in the photographic industry where it is necessary to provide a standard of illumination; and in other applications where known light sources are needed, it is highly desirable to be able to have a standard source of illumination which can be relied upon for a prolonged period of time to emit a radiation level which varies only Slightly over the time of its use. Such a standard illumination source is generally used as a standard for comparison or for control work; and it is, of course, necessary that it can be relied upon to remain relatively constant over an extended period of time thus eliminating periodic calibrations.

Incandescent lamps with tungsten filaments are generally used for standard sources of illumination. The filament current heats the filament, and the thermally emitted photons comprise the usable radiation, as described by Plancks radiation formula. The radiance of interest (i.e., the radiated energy per unit area over a particular wavelength interval) is a function of filament temperature, specifically for small temperature changes,

R F T where:

R=radiance of interest T=filament temperature K constant n=constant depending on wavelength interval, typically Heretofore, conventional practice has been to maintain constant radiance by maintaining constant filament temperature by means of a constant filament current (or con stant filament voltage). In radiation equilibrium, over small temperature ranges where:

I =filament current =filament resistance K =constant.

The short-term stability of such a radiation source is determined by the ability to control filament current (or 3,522,464 Patented Aug. 4, 1970 voltage). Combining (1) and (2) and setting n=5 yields AR pk Ir I or, for every 1% fluctuation in filament current there is a 2.5% fluctuation in radiance.

The long-term stability of such a conventional source is further degraded by evaporation of the tungsten from the filament. Two mechanisms are involved. First, evaporation from the filament will increase its resistance and result in increased radiance for a constant filament current (Equations 1 and 2). Second, the tungsten which is evaporated condenses on the cool lamp envelope thereby decreasing the fraction of radiation transmitted through the envelope. Thus, it will be seen that over a period of time the conventional incandescent lamp is not an invariable source of radiation and it varies in a way for which there is no accurate compensation.

It would, therefore, be an advance in the art to make available a source of radiation in the range specified which is less sensitive to imperfections in the external control circuitry and is capable of emitting constant radiance over an extended period of time without calibration and without any appreciable variation in the radiance emitted from the filament. Since any such device which uses a tungsten filament is subject to variations brought about by the condensation of the tungsten on the envelope, it would also be desirable to have a tungsten filament lamp which had means incorporated in it which would permit the direct determination of the degree of tungsten condensation on the envelope.

It is, therefore, a primary object of this invention to provide a radiation source together with suitable circuitry for a standard emission of radiation, such a source not requiring periodic calibration.

It is another object of this invention to provide a standard radiation source which is more stable than conventional tungsten lamps but which does not require the use of a more stable external control circuitry. It is another object of this invention to provide a standard radiation source of the character described which is flexible in its application to many different uses. It is yet another object of this invention to provide such a source in one modification which incorporates a means for the ready determination of the amount of tungsten which has been deposited on the envelope and thus a ready and accurate determination of the decrease in observed radiance. Other objects of the invention will in part be obvious, and will in part be apparent hereinafter.

The invention accordingly comprises the features of construction, combination of elements and arrangement of parts, which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.

For a further understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings in which:

FIG. 1 illustrates an incandescent lamp suitable for use in the standard radiation source of this invention;

2 is a typical circuit controlling the lamp of FIG. 3 illustrates a modification of the lamp of FIG. 1, designed to allow estimating the amount of tungsten deposition on the envelope;

FIG. 4 is a cross-section of the lamp through line 44 of FIG. 3; and

FIG. 5 is a detail drawing of the movable shield of the apparatus of FIG. 4.

The standard light source or radiation source of this invention employs an incandescent lamp having a tungsten filament enclosed in an evacuated envelope. The tempera ture of the filament is sensed in terms of the electron current to the anode electrode; and through the use of a servo loop, the anode current can be held constant by correcting the voltage across the tungsten filament. Hence, there is no longer the limitation on the lamp that the change in temperatures changes the resistance which, in turn, changes the output of the lamp.

In FIG. 1 a lamp such as is suitable for the standard light source of this invention is shown. It will be appreciated that in such a lamp the flow of anode current (i.e., the rate of capture of electrons thermally emitted from the filament) is a very sensitive indicator of the filament temperature produced by resistance heating. Specifically, for small temperature ranges a= sF r where I anode current K =constant Equation 2 still holds true for radiation equilibrium, and this leads to the short-term performance equation for the invention (for ni=5) 9am. R 18 L,

or for every 1% fluctuation in anode current there is a 0.28% fluctuation in radiance. Clearly this offers improved stability over the more conventional radiation source without resorting to more complicated external control circuitry.

Furthermore, it will be clear that through suitable masking the anode current will be a function of the material properties and temperature of a small area of the filament. Similarly, through suitable masking the radiance of interest is also a function of the local material properties and temperature of the same or similar area on the filament. Both of these parameters are independent of the resistance and size of the filament. Therefore, control of filament radiance through external control of anode current is quite independent of filament evaporation.

Turning to FIG. 1, the incandescent lamp of the radiation source of this invention comprises an envelope 10 containing in the evacuated space 13 defined thereby an incandescent filament 11 which is in turn, connected to a suitable voltage source 12. Within the tube there is also supplied an anode 14. The electrons that boil off from the tungsten filament 11 are driven to the anode and constitute the anode current. This filament may be formed of tungsten wire, or it may be a single crystal of tungsten. The rate at which the electrons reach the anode is related to the filament temperature and the electric field between filament and anode. The electric field, E, must be above a certain limit, otherwise changes in current are obscured.

It follows then that, if it is possible to control the temperature of the filament, it is possible to provide a standard source of illumination. Control of the filament temperature is achieved in the radiation source of this invention through the control of the anode current, which in turn is done by a suitable circuitry containing a servo amplifier and feedback arrangement so that the anode current may be kept constant.

The complete standard light source of this invention is shown in FIG. 2. An important component of the circuitry of the light source is the servo amplifier. If the input lead of the servo amplifier is maintained at zero or at some fixed potential, the servo amplifier will attempt to maintain the input terminal at a fixed potential. If, for example, there occurs a drop in the anode current in the lamp 10, the anode potential rises and any such change is communicated in part to point A of the lamp source circuit and appears as an error signal to the servo. That is, it appears as a deviation from the set point. The servo then changes its output in a. sense that it minimizes the change. The output voltage of the servo amplifier tends to increase, the filament of the lamp tends to get hotter, and the anode potential of the lamp returns to equilibrium conditions. If the filament gets too hot, it will, of course, reverse the process described.

Lamps, such as illustrated in FIG. 1, have been operated in the standard light source represented by the circuit of FIG. 2 to indicate the usefulness of the constant light source of this invention. Operation was carried out under two conditions of filament temperature, namely, 2,000 K. and 2,250 K. Anode currents were 50 microamps and 1.1 milliamps, respectively, and within the time they were operated, there were no observable dilferences in energy radiated. The experiment of evaluation consisted of maintaining the anode current almost constant. This was achieved by comparing a reference voltage with a voltage proportional to the anode current and feeding an error signal to a chart recorder of 0-1 mv. sensitivity. A mechanical connection was made through a gear box from the recorder motor to a variac to the lamp filament. In this way, a change in anode cun'ent caused a change of filament voltage in the sense that the anode current tended to remain constant. Under the most undesirable situation the luminous output change was not more than 0.0025%. Thus, it will be seen that the deviations in radiation emitted were far less than that which can be expected from a normal incandescent lamp, such as now used for this purpose. In actual use of the light source of this invention, a DC system would be preferred.

Inasmuch as tungsten deposition on the envelope always occurs when a tungsten filament is heated, it is desirable to be able to determine when the inside of the envelope defining the lamp of the light source has been coated with sulficient tungsten to make it no longer usable as a standard source of radiation. It will be appreciated that the deposition of tungsten vapor in the form of a very thin film on the internal walls of the envelope decreases the amount of radiation which can be transmitted through the envelope.

A device for determining the amount of tungsten which has built up on the internal walls of the envelope is illustrated in FIGS. 3, 4, and 5.

In the lamp of this light source, there is provided within the envelope 10 (which terminates in a suitable base 20) a shield 22, which has a front port 23, through which radiation may pass and be transmitted through a portion of the envelope 10. From the examination of the crosssectional representation of FIG. 4, it will be seen that the tungsten as it evaporates builds up a thin film 15 on the internal walls of the tube 10. Opposite port 23 in the shield 22 is a second port 25, which normally is closed to prevent vaporized tungsten from striking the other side of the internal walls of the envelope 10. The closure over the port is conveniently a movable plate 26, which is mounted on a pin 27. This is shown in elevational view in FIG. 5.

After the lamp has been operated for a period of time, it is possible to move the cover 26, which normally protects the port 25, to one side or another out of the position which it normally occupies and to permit radiation from the filament 11 to pass through port 25 and be transmitted through that part of the envelope wall which corresponds to the port 25. The closure 26, which is normally blocking port 25, may be moved to either side if the closure itself is formed of a magnetic materia such as nickel. Placing a magnet to one side or the other of the closure 26 will draw it upwardly and out of position so that the port 25 is open and free to transmit radiation to the envelope 10 and through it to a measuring device, such as a photoelectric tube. By comparing the amount of radiation which is permitted to pass through the area 15 with that which is permitted to pass through the corresponding area 16 on the opposite side, it is possible to determine to what extent the tungsten has built up as a film on the area 15 and hence to determine whether or not there has been sufiicient tungsten film to alter the amount of radiant energy transmitted through the envelope. Thus, there is built into the lamp shown in FIG. 3 a convenient way of evaluating the accuracy of the lamp and its ability to act as a component of a standard light source constructed in accordance with this invention.

The incorporation of the modified lamp shown in FIGS. 3 through 5 in the circuitry of FIG. 2, therefore, provides a standard illumination component which may be periodically checked for changes due to tungsten deposition on the envelope walls and which is normally accurate in its emission of a standard quantity of radiant energy. Because the circuity of FIG. 2 embodies the constant monitoring of the anode current, it, in effect maintains a constant filament temperature and hence provides a constant source of radiant energy.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efiiciently attained, and since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description,, or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. A stable source of radiant energy, comprising in combination (a) an incandescent lamp having an envelope defining an evacuated space containing therein an incandescent tungsten filament;

(b) an electrode disposed within said envelope and spaced from said filament;

(c) a radiation shield surrounding said filament, said shield having first and second ports adapted to pass radiation for transmission through said envelope of said lamp and having closure means associated with at least said second port adapted to prevent the passage of radiation therethrough; (d) voltage reference means associated with said electrode; and I (e) servo amplifier means associated withsaid filament,

connected to said voltage reference means and responsive to changes in current through said electrode so that the voltage across the filament is variable for maintaining said current substantially constant whereby the radiation emitted by said source remains stable. 2. A stable source of radiant energy in accordance with claim 1 wherein said closure means is a magnetically actuatable plate pivotally mounted on said shield.

References Cited UNITED STATES PATENTS 1,215,029 2/1917 Jones 240-4621 1,562,897 11/1925 Lissel 313-117 1,728,814 9/1929 Van Liempt 1481.6 1,850,467 3/1932 Nakken 313-117 X 2,149,080 2/1939 Wolff 315-107 X 2,627,035 1/1953 Ball 250-97 3,219,929 11/1965 King 315-107 X JAMES w. LAWRENCE, Primary Examiner E. R. LAROCHE, Assistant Examiner U.S. Cl. X.R. 

